Presently, there is an effort by many electronics manufacturers to achieve ever higher levels of performance by greater integration of individual components in a single assembly. One technique for achieving large scale integration is to interconnect each of a plurality of Very Large Scale Integrated (VLSI) circuit chips by way of a silicon substrate, or the like, rather than to package the devices and then connect the packaged devices via a conventional circuit board. The most common method for attaching an integrated circuit chip to such a silicon substrate is to employ a technique known as "flip-chip" bonding, which is practiced by first attaching a solder bump to each of a plurality of bonding areas on the surface of the substrate and to each of a plurality of bonding areas on the undersurface of the chip. The chip is then placed on the substrate so that each solder bump on the chip is aligned with a corresponding bump on the substrate. Thereafter, the integrated circuit chip is thermocompressively bonded to the substrate.
In practice, there are usually a large number of connections to be made between each integrated circuit chip and the substrate. As a consequence, each integrated circuit chip has a large number of closely spaced solder bumps, usually arranged in a rectangular array of rows and columns, as does the substrate. In U.S. Pat. No. 4,980,971, issued on Jan. 1, 1991, to M. K. Bartschat et al., and assigned to AT&T Bell Laboratories, there is disclosed a system for precisely placing a semiconductor chip on a substrate with the aid of a robotic arm. To achieve precise alignment, a first television camera is carried by the robotic arm and is trained downward for capturing the image of the substrate to establish the position of each of a pair of fiducials thereon. A second camera, which is stationary, is trained to look upward at the robotic arm to observe the undersurface of a chip carried by the arm to establish the position of each of a pair of reference points on the chip. The output of each of the first and second cameras is processed by a machine vision processor to establish the precise position of the solder bumps on the chip and the position of the bonding areas on the substrate to allow the robot to precisely place the chip on the substrate.
The Bartschat et al. system is capable of achieving a placement tolerance on the order of 10 microns, which is satisfactory in most instances. However, there are currently integrated circuits which have such a large number of closely spaced solder bumps that a placement accuracy on the order of 3 microns is required. Heretofore, no known placement system could achieve such a high placement accuracy.
Thus, there is a need for a technique for precisely placing an article, such as an integrated circuit chip, on a substrate with a very fine placement accuracy.